mountlib: correctly daemonize for compatibility with automount - #5593

This patch will:
- add --daemon-wait flag to control the time to wait for background mount
- remove dependency on sevlyar/go-daemon and implement backgrounding directly
- avoid setsid during backgrounding as it can result in race under Automount
- provide a fallback PATH to correctly run `fusermount` under systemd as it
  runs mount units without standard environment variables
- correctly handle ^C pressed while background process is being setting up

lib/daemonize/daemon_unix.go

@@ -0,0 +1,59 @@
+// Daemonization interface for Unix platforms (implementation)
+
+//go:build !windows && !plan9 && !js
+// +build !windows,!plan9,!js
+
+package daemonize
+
+import (
+	"os"
+	"syscall"
+
+	"github.com/rclone/rclone/fs"
+)
+
+// StartDaemon runs background twin of current process.
+// It executes separate parts of code in child and parent processes.
+// Returns child process pid in the parent or nil in the child.
+// The method looks like a fork but safe for goroutines.
+func StartDaemon(args []string) (*os.Process, error) {
+	if fs.IsDaemon() {
+		// This process is already daemonized
+		return nil, nil
+	}
+
+	env := append(os.Environ(), fs.DaemonMarkVar+"="+fs.DaemonMarkChild)
+
+	me, err := os.Executable()
+	if err != nil {
+		me = os.Args[0]
+	}
+
+	null, err := os.Open(os.DevNull)
+	if err != nil {
+		return nil, err
+	}
+	files := []*os.File{
+		null, // (0) stdin
+		null, // (1) stdout
+		null, // (2) stderr
+	}
+	sysAttr := &syscall.SysProcAttr{
+		// setsid (https://linux.die.net/man/2/setsid) in the child process will reset
+		// its process group id (PGID) to its PID thus detaching it from parent.
+		// This would make autofs fail because it detects mounting process by its PGID.
+		Setsid: false,
+	}
+	attr := &os.ProcAttr{
+		Env:   env,
+		Files: files,
+		Sys:   sysAttr,
+	}
+
+	daemon, err := os.StartProcess(me, args, attr)
+	if err != nil {
+		return nil, err
+	}
+
+	return daemon, nil
+}